The instant invention relates to methods of producing biaxially oriented polymer product and to the product produced thereby. More particularly, the instant invention is directed to methods of producing such a product by continuous forging, wherein polymer feedstock is advanced between a pair of opposed belts.
Biaxially oriented polymers, such as the polypropylene polymers sold under the trademark BEXOR, have numerous advantages over unoriented polymers in that they have higher strength, improved stiffness, increased toughness and superior thermoformability. Biaxially oriented polymers have been successfully produced using the techniques disclosed in U.S. Pat. No. 4,282,277, the disclosure of which is incorporated herein by reference, assigned to the assignee of the instant invention, wherein a tubular product is initially produced by hydrostatic extrusion using a mandrel in combination with a conical die. The tubular product is then cut, flattened and annealed utilizing expensive flattening and annealing equipment. While the mandrel-conical die approach results in excellent biaxial orientation of structures, it is necessary to expend considerable effort to remove residual curvature in the resulting sheet. Even when the curvature has been removed by reconfiguring the extruded tube into a flat sheet, residual and unbalanced stresses and strains in the original tube tend to subsequently reassert themselves, resulting in difficulties when using the sheet.
In addition, the cost of the biaxially oriented sheet produced in accordance with the teachings of U.S. Pat. No. 4,282,277 is relatively high, since only a single billet may be extruded at a time and intermittent production results in high yield losses. In addition, it has proved expensive and difficult to keep the mandrel properly aligned within the die and to produce uniform sheet during the extrusion process.
In order to avoid the expense of first forming a tube and subsequently slitting and flattening the tube to form a biaxially oriented sheet, the inventors of the instant invention explored extruding polymers while in the solid state utilizing dies with protrusions therein as is set forth in U.S. patent application Ser. No. 806,994, filed Dec. 9, 1985. Utilizing protrusions within flat dies overcomes the non-uniformity which ordinarily occurs in flat dies due to friction between the workpiece and the die. While this approach results in a product which does not have to be slit and flattened before producing sheet, the process is an intermittent one, wherein the sheets are produced one at a time, as opposed to a continuous process. Moreover, utilization of a stationary die requires a very large machine, which can withstand required extrusion pressures on the order of 10,000 psi (703 kg/cm.sup.2) or more. In actual experiments, pressures of 7,000 psi (492 kg/cm.sup.2) were required for flat dies having protrusions whereas pressures of only 1200 psi (84.4 kg/cm.sup.2) were required for twin-belt machines. Due to high extrusion pressures, flat die machines require an initial capital outlay much greater than twin-belt machines.
While biaxially oriented sheet can be produced by platen forging and cross-rolling, each of these processes has the drawback of being a batch process as opposed to a continuous process and therefore has serious size and economic limitations. Consequently, it is difficult to produce elongated sheets of material. In addition, with a sheet forged between parallel platens, the sheet must be produced from a circular blank in order to have uniform biaxial orientation. The blank must then be trimmed which adds an additional step and wastes material. While sheets produced by crossrolling have a roughly rectangular shape, they exhibit wavy surfaces resulting from non-uniform elastic springback of sheet emerging from the roll nip and consequently are not necessarily suitable for subsequent shaping and forming. Moreover, due to short deformation times in cross rolling, the resulting elastic springback leads to a reduction of desirable properties such as stiffness.
Pursuing the opposed-belt concept, the inventors of the instant invention investigated utilizing a twin belt annealing press of the type generally used in the manufacture of items such as particle board, wherein wood particles or fibers are pressed together in the presence of a bonding agent to create sheets of material. This type of process is practiced at lower pressures than those required to biaxially orient polymers and was proved by tests to be unsatisfactory. The inventors also explored twin-belt machines used for producing metal sheet or slab by continuous casting. However, continuous casting machines only require sufficient pressure to retain the melt being processed thereby. In these types of machines, molten material is deposited between a pair of converging belts and spreads laterally and longitudinally between the belts in order to continuously produce a metallic sheet product. In view of the inadequacy of these approaches, there appears to be little promise in utilizing existing opposed belt approaches for biaxially orienting feedstock to produce biaxially oriented polymer sheet.
As is set forth in parent applications, 933,951 and 806,994, biaxial orientation of thermoplastic sheet in continuous twin belt presses requires at least three major steps. The first is the transport of thick plastic slabs by belts through an angled deformation zone where the plastic slabs are squeezed so that the material therein flows both parallel and perpendicular to the extent of the belts thereby forming biaxially oriented sheet. Secondly, the biaxially oriented sheet is passed through a zone where the belts are parallel and wherein the molecular structure of the material of the sheet is heat set or annealed. Finally, the sheet which has been annealed is transported through a cooling zone in order to reduce the temperature of the sheet to a level below its heat deflection temperature so that the sheet remains flat.
The first step in which the narrow slab is converted to a wide sheet causes the surface area of the plastic to increase. This requires an isochoric or roll supported belt system which allows the belts and product to move through the press. Isochoric presses for biaxially orientation of plastic sheets have several disadvantages. First of all, belts have limited life due to bending and cracking at the points where the rolls support the belts in the high pressure deformation zone. Moreover, heat transfer from the hot plastic sheet to the machine cooling zone is poor, since heat flows from the belts through individual roll contact points or lines to the cold stationary press platens. In that heat transfer controls production rates of biaxially oriented sheet in twin belt presses, very long machines (and belts) must be used for the production as to be economical. Consequently, machines and belts can become very expensive. In addition, if a special finish such as a glossy or textured finish is required on the biaxially oriented plastic sheet, the finish must be applied to the working surfaces of both belts which can add enormous additional expense to the cost of the belts.
In view of these considerations, and other considerations, there is a need for improvement in isochoric processes and other processes accompanying the isochoric processes and there is a need for apparatus to accomplish these improvements and processes.